1. Field
This disclosure relates generally to wireless communication and more specifically, but not exclusively, to techniques for adding hybrid ARQ to WLAN protocols with MAC based feedback.
2. Introduction
In Automatic Repeat reQuest (ARQ) schemes, an error detection code is added to an encoded data packet before transmission to a receiver. The receiver attempts to decode the packet and uses the error detection code to determine whether or not it has been successful. If successfully decoded, the receiver sends an acknowledgement (ACK) back to the transmitter. Otherwise, the receiver sends a negative acknowledgement (NACK) back to the transmitter requesting retransmission of the data packet. A common example of an error detection code is a cyclic redundancy check (CRC) code.
In addition to a error detection code, the data packet may be additionally protected with an error correction code. Error correction coding generally involves generating several symbols for each bit in the data packet. The symbols include “systematic symbols” and “parity symbols.” The systematic symbols represent the bits in the data packet and the parity symbols provide redundancy. This redundancy may be used by the receiver to correct errors in the transmission due to disturbances in the communication channel. The error detection code may be used to determine whether or not the receiver was able to successfully decode the data packet (i.e., correct all the transmission errors). Based on this determination, the receiver provides feedback to the transmitter using an ACK based protocol. This technique is often referred to as hybrid ARQ (HARQ).
Incremental redundancy is a more sophisticated type of HARQ which designed to be more robust in environments with fluctuating channel conditions. When incremental redundancy is used, the first transmission contains only systematic symbols with error detection. If the transmission is successfully decoded by the receiver, an ACK is sent back to the transmitter. If the receiver is unable to decode the transmission, then a second transmission will contain a selected number of parity symbols with error detection. Decoding is again attempted at the receiver by combining the systematic symbols with the transmitted parity symbols. If a retransmission is required, the transmitter sends additional parity symbols with error detection. This procedure is repeated until all the parity symbols are transmitted.
Recently, there has been a tremendous growth in the deployment of Wireless Local Area Networks (WLAN), such as IEEE 802.11 and the like. As these networks become more prevalent and begin working in closer proximity to one another, frequency reuse will be increasingly important to ensure efficient medium utilization. Such frequency reuse will cause the signal-to-noise ratio (SNR) at the receiver to fluctuate rapidly. The current convolutional codes are not robust to such variations in SNR, and as a result, a large fraction of data packet transmissions will have to be discarded by the receiver, thereby reducing the efficiency of the network.
Accordingly, there is a need in the art to add HARQ to a wireless transmission protocol for a WLAN, such as IEEE 802.11 and the like. However, in many WLAN, where the ACK based protocol is implemented in the MAC layer, the current techniques do not support the implementation of HARQ.